Carbon electrodes used in electric furnaces, such as in the electrothermal production of phosphorus, are consumed in use. A substantially continuous operation of furnace usually is required for economic product production and hence means for continually feeding the electrodes into the furnace without interruptions is necessary. This typically has been achieved by fabricating electrode segments with suitable threaded sockets at both ends, so that a plurality of electrode segments can be connected to each other under tension through a cylindrical threaded connector, known as a nipple. In this way, a new electrode segment may be joined to an electrode segment being consumed through the nipple allowing for a continuous feed operation.
When such an electrode joint has been employed it has been found that stresses concentrated at the flank portion of the threads nearest the abutting end faces of the electrode segments often cause loosening of the joint with consequently-increased electrical resistance at the joint and hence increased furnace power requirements. In some cases, the stresses may be so severe as to cause splitting or cracking of the electrode sections.
Loosening of the joints in this way necessitates post torquing of the joints from time to time to retighten the same with attendant labor costs and lost production time, which may abount to as much as 2 hours per week for each electrode or about 3 1/2 percent of the total operating time.
The stresses arise largely from thermal expansion stresses brought about by the differences in thermal expansion coefficient of the materials of construction of the electrode segments and the nipple. Thus, the electrode segments usually are made of carbon while the nipples usually are made of graphite, the latter material having about half the thermal expansion coefficient of the carbon.